1. Field of the Invention
The invention relates to a sensor for static and/or dynamic monitoring of at least one physical quantity of a liquid or flowable medium, and particularly relates to a pressure sensor with a housing and a measurement cell in which one side of the measurement cell is in contact with the medium to be monitored. The measurement cell converts the physical quantity which is to be monitored into a proportional measurement signal. In addition, the invention relates to a measurement cell, especially a ceramic pressure measurement cell for use in a sensor in which the measurement cell has a base body and a membrane which is connected to the base body such that one side of the membrane is in contact with the medium to be monitored. The invention is also drawn to a process for producing a measurement cell.
2. Description of Related Art
Conventionally, sensors used for monitoring of a medium or the property of a medium acquire a physical measurement value and then convert this measurement value into an electrical signal which is then relayed to a central evaluation unit, which is connected to one or more sensors via electrical lines, or to a decentralized evaluation unit which is directly connected to the sensor. Sensors, often called fluid sensors, generally have an electronic unit in which the measured values, which have been delivered from the measurement cell, are converted and, optionally, processed or evaluated.
For these sensors there are a plurality of applications. Thus, for example, the pressure, the temperature, the fill level, the flow rate or the flow amount of liquid or solid media, as well as a bulk material, can be monitored or measured. There are different measurement principles for determining the individual parameters. For example, fill level sensors can work using the capacitive or the hydrostatic measurement principle.
Pressure sensors are used to monitor and measure the system pressure in hydraulic and pneumatic applications. In such a system, the pressure to be monitored can also be a negative pressure so that the sensors are instead vacuum sensors. One application of these pressure sensors is, for example, in the foodstuffs industry in which the pressure of various media, especially various liquids, is monitored or measured. For the instant invention, there are a plurality of different embodiments, depending on the application, with the structure and the design of the pressure sensors differing depending on the expected maximum nominal pressure and the type of medium which is to be monitored.
Known pressure sensors conventionally have a cylindrical pressure measurement cell which includes a base body and a membrane which are held at a defined distance from one another and are tightly, hermetically joined to one another by a connecting material, e.g., a brazing solder. Then, the side of the membrane in contact with the medium to be monitored undergoes a deflection which is proportional to the pressure of the medium and which can be detected by means of an electromechanical converter which is located on the other side of the membrane, see for example German Application Nos. 44 16 978 C2 or 196 28 551 A1 which corresponds to U.S. Pat. No. 5,824,909.
In the first type of pressure sensors, the capacitive measurement principle is used as the measurement principle. Specifically, the inner surfaces of the membrane and the base body are coated with electrodes to form a measurement capacitor with a capacitance which is a function of the camber of the membrane, i.e., a measure for the pressure on the membrane. In practice, pressure measurement cells of a ceramic material are generally used since such cells have high measurement accuracy which remains stable over a very long time. On the side facing away from the medium, the pressure measurement cell has an electronic circuit which converts the capacitance of the measurement capacitor into an electrical signal dependent on pressure and makes the signal accessible to further processing or display via electrical connecting leads.
In addition to these pressure sensors, which employ the capacitive measurement principle and include two electrodes used as the electromechanical converter, there are also pressure sensors which have resistance strain gauges (DMS) or pressure-sensitive resistors or DMS resistors. In these pressure sensors, the DMS resistors are applied to the side of the membrane facing away from the medium and the resistance value of the DMS resistors depends on the camber of the membrane. Therefore, the resistance is a measure of the pressure on the membrane. In these pressure sensors or pressure measurement cells, a separate base body opposite the membrane is not necessary in terms of measurement engineering. Therefore, there are pressure sensors or pressure measurement cells with DMS resistors which are monolithic, i.e., the base body and the membrane are constructed in one piece. In this embodiment, the membrane is generally on the side of the base body which has a blind hole in the area of the membrane, which side faces away from the membrane. The base body has a pot-shape with the bottom of the pot being formed by the membrane and the open side of the pot facing the membrane. In addition, it is also possible for the membrane to be located on the side of the base body facing the medium.
Regardless of the type of measurement principle of the pressure sensor, in the known pressure sensors there is quite often the problem that the membrane and the measurement cell must necessarily come into contact with the medium which is to be monitored. Therefore, for corrosive media the known sensors cannot be used or can only be used at great cost. The problem is that such measurement cells have only limited resistivity to the corrosive media, such as sodium hydroxide solution, phosphoric acid, sulfuric acid or chromic acid, particularly when such cells are used in conjunction with an elevated temperature of the corrosive medium.
In the prior art, various attempts have been made to avoid the aforementioned disadvantages. One possibility for increasing the resistivity of a ceramic measurement cell to a medium is to use a measurement cell of high-purity ceramic. Conventional, relatively economical measurement cells consist of 96% ceramic which does meet the normal mechanical stresses of the measurement cell, but has only limited media resistivity to the corrosive medium. The resistivity of the measurement cell to the corrosive media can be increased by using a high-purity ceramic, for example 99.9% ceramic or sapphire ceramic. However, these measurement cells are relatively expensive so that they are only worthwhile in special individual cases.
German Patent 36 29 628 C2, which corresponds to U.S. Pat. No. 4,617,607, and German Patent 39 12 217 C2, which corresponds to U.S. Pat. No. 5,076,147, disclose pressure sensors in which on the side of the membrane facing the medium a thin metal membrane is pretensioned or a silicon carbide layer is applied to the membrane. When using an additional metal membrane it is disadvantageous in that the metal membrane changes the deflection characteristic and, therefore, the sensitivity of the sensor so that the use of a metal membrane in pressure sensors is not suited for a relatively low nominal pressure. When a layer of silicon carbide is applied, by means of gas phase deposition, a problem arises in that deposition of silicon carbide occurs at very high temperatures of roughly 1000° C. which can lead to damage to the membrane or the measurement cell. In addition, in the known coating process there is also the danger that the layer of silicon carbide has individual pores such that reliable protection of the membrane is not ensured. The problem of the occurrence of pores in the layer of silicon carbide particularly acute when a relatively thin layer is desired.